In a portable small electronic apparatus, employed is a 2 power source method of using an AC adapter utilizing a commercial (AC) power source as a main driving power source, and in an environment where the commercial power source (AC power source) cannot be utilized, using a battery as a driving power source. In this case, as the battery, a rechargeable secondary battery which can be repeatedly used is widely employed. Further, in some cases, the secondary battery is electrically charged with electric power supplied from the AC adapter.
In a product including an AC adapter and a battery as standard equipments such as a notebook type PC, the PC main body generally includes a battery charging function. However, in a small image printing apparatus such as an ink jet printing apparatus, including only an AC adapter as a standard equipment, a battery as a secondary driving power source is an optional equipment. Accordingly, in such printing apparatus, if the battery charging function which is necessary only in use of the battery is included in the apparatus main body as in the case of the notebook type PC, unnecessary cost increase and upsizing are imposed on a user who does not require a battery. Accordingly, in some cases, the battery charging function is provided as a separate unit from the printing apparatus main body, and is handled as an optional equipment similar to the battery. Note that to avoid troublesome removal of the battery from the apparatus main body and setting of the battery in the charger upon every electric charging, a known arrangement is that the battery is attached to the electric charging unit which is attachable/removable to/from the apparatus main body and which has an electric power relay function to relay-output electric power from the battery to the apparatus main body.
Generally, to electrically charge the battery in a short period, a large amount of electric power is required. Further, to simultaneously perform driving of the apparatus and electrically charging of the battery, it is necessary to use a large capacity AC adapter and high-rated current components. Accordingly, the cost and size of the apparatus are greatly increased. To avoid such cost increase and upsizing, employed is electric charging control to quickly charge the battery when the apparatus is in a power-off status or stand-by status with a small amount of electric consumption, or electric charging control to perform quick charging when the amount of electric consumption is small while to perform slow electric charging with a lowered charging current when the amount of electric consumption is large.
As described above, in the arrangement that the battery charging function is provided as a separate unit from the apparatus main body and the electric charging unit and the apparatus main body are driven with the AC adapter as a main driving power source, the electric charging unit is provided with means for detecting a consumed current in the apparatus main body. When the consumed current is less than a predetermined value, quick electric charging is automatically performed.
However, for detection of the consumed current in the apparatus main body by the charging unit, it is necessary to add the means for detecting the consumed current with high accuracy to the electric charging unit. Thus, the size and cost of the electric charging unit are increased. Further, as a voltage drop occurs due to resistance for the detection of consumed current, the apparatus when driven with the battery enters a low-battery state indicating reduction of battery residual capacity (electric power) ahead of time by the voltage drop. Thus a driving period per one electric charging is shortened.